Microlithography is used for the manufacture of micro-structured components such as for example integrated circuits or liquid crystal displays (LCDs). The microlithography process is performed in a so-called projection exposure apparatus which includes an illumination system and a projection objective. The image of a mask (also called a reticle) which is illuminated via the illumination system is projected via the projection objective onto a substrate (for example a silicon wafer) which is coated with a light-sensitive coating (for example a photoresist) and set up in the image plane of the projection objective, whereby the mask structure is transferred to the light-sensitive coating of the substrate.
In current types of microlithography objectives, in particular immersion objectives with a numerical aperture value (NA) not exceeding 1.0, there is more and more a desire to use materials of a high refractive index, in particular for the last optical element on the image side. In this context, a refractive index is referred to as being high if it exceeds the value for quartz at the given wavelength, i.e., approximately 1.56 at a wavelength of 193 nm. A number of materials are known whose refractive indices at DUV (deep ultraviolet) and VUV (vacuum ultraviolet) wavelengths (<250 nm) are larger than 1.6, for example magnesium spinel (MgAl2O4), which has a refractive index of approximately 1.87 at 193 nm, or lutetium aluminum garnet (Lu3Al5O12), which has a refractive index of about 2.14 at 193 nm.
An issue encountered in using these materials as lens materials is that due to their cubic crystallographic structure they exhibit intrinsic birefringence of increasing magnitude the shorter the wavelength. For example in magnesium spinel, measurements have shown a retardation of 52 nm/cm due to intrinsic birefringence, and for lutetium aluminum garnet a retardation of 30.1 nm/cm. The term “retardation” in this context means the difference between the respective optical path lengths for two orthogonal (i.e., mutually perpendicular) states of polarization.